As the degree of integration of a semiconductor integrated circuit has increased 4-fold for 3 years, a dynamic random access memory (hereinafter “DRAM”) having the memory capacity of more than one gigabit has been developed. To produce such a large capacity DRAM, there is a need for the development of a photosensitive polymer and a photoresist composition capable of forming a fine photoresist pattern whose line width is of 0.18 μm.
Generally, the photolithography process for the semiconductor manufacturing process includes the steps of a) uniformly coating a photoresist composition containing a photosensitive polymer and a solvent on a semiconductor substrate, b) soft-baking the coated photoresist composition to drive off the solvent, thereby forming a resist film on the substrate, c) exposing to an exposure light source, a photo-mask(reticle) and the substrate on which the resist film is formed, thereby repeatedly projecting the reduced-image of the photo-mask to the resist film; d) selectively removing the exposed resist film using a developer; e) hard-baking the developed resist film for rigid sticking of the film to the substrate; f) etching the substrate according to the hard-baked resist film pattern; and g) stripping the unnecessary resist film after the etching step.
The photoresist composition for the photolithography process must have properties such as a high resolution, a good optical sensitivity and a good transparency with respect to the exposure light, and also must have properties such as a good contrast, a fast photo-speed, a good thermal stability, adhesiveness and etch resistance. The photosensitivity represents the degree of reaction of the photoresist composition in response to the exposure light. If the photosensitivity is fine, the amount of the photoresist composition reacting with the exposure light per unit hour is increased and then the productivity can be enhanced. In order to increase the photosensitivity, there is introduced a chemically amplified photoresist composition containing a photosensitive polymer and a photo-acid generator. If the chemically amplified photoresist composition is exposed to the light, the photo-acid is generated from the photo-acid generator to form a latent image in the photoresist composition. The generated photo-acid is activated at the heating process after the exposure, to cause a) a cross-linking reaction of the photosensitive polymer contained in the photoresist composition or b) a depolymerization or deprotection of the main chain or a functional group substituted to the main chain of the photosensitive polymer. In turn, the photo-acid can be generated as a by-product of the above-mentioned cross-linking reaction, depolymerization or deprotection, and then a chain reaction therefrom occurs to enhance the photosensitivity of the photoresist composition. The resolution is defined as a minimum size of the fine circuit pattern that can be formed by the photoresist under the most suitable process condition, and is an important index for evaluating the photoresist. The resolution(R) is mathematically defined by (κλ/NA), wherein κ is a parameter dependent on the process condition and an inherent physical property of the photoresist, λ is a wavelength (nm) of the exposure light and NA is a numeral aperture of the lens. As shown in the above expression, the high resolution can be accomplished by using a light source of a shorter wavelength or the photosensitive polymer and the photoresist composition having a low parameter (κ), at the exposure process.
Historically, early in the 1980s, the G-line (436 nm) exposure process had been introduced using a high-pressure mercury lamp to form a circuit pattern having a resolution of 1 μm and the I-line (365 nm) exposure process had been introduced to form a circuit pattern having a resolution of 0.5 μm or less. Thereafter the exposure technology using a shorter wavelength of 300 nm or less, for example KrF eximer laser using a light of 248 nm wavelength, had been developed to mass-produce 256M DRAM having the resolution of less than 0.5 μm. A step-and-repeat type aligner, which is usually called “a stepper” is used as an equipment for the exposure process. The stepper is used according to the exposure light source, for example, G-line(436 nm), 1-line(365 nm), ArF(193 nm) eximer laser, KrF(298 nm) eximer laser and so on. The exposure equipment using X-ray or EUV (Extreme Ultra Violet) as the exposure light source is under the investigation and development.
As described above, the photoresist composition for the photolithography process using a light of a short wavelength as an exposure light source must have a good physical property such as the good transparency, good photosensitivity and high resolution. However, a conventional composition consisting of quinonediazide photo-activation compounds and phenol-novolak resin, which is used in the conventional G-line(436 nm) or I-line(365 nm) exposure process, absorbs the exposure light of 300 nm or less and has a low transparency. Also, the patterns formed by the conventional G-line(436 nm) or I-line(365 nm) exposure process are not stable, and the steep patterns cannot be formed. Therefore, there is a demand for a chemically amplified photoresist composition having an excellent physical property even in case of using the exposure light source of 300 nm or less.
Among the chemically amplified photoresist composition, the negative photoresist composition includes a binder resin that is soluble in an alkali aqueous solution, a cross-linking agent, a photo-acid generator and a solvent (Jour. Vacuum Science Technology., Vol. B6, 1988). The photo-acid generated from the photo-acid generator operates as a catalyst for activating the cross-linking agent so that the binder resin becomes insoluble and the negative circuit patterns are formed in the succeeding developing process. When the conventional photoresist composition including a novolak resin as the binder resin, a melamine cross-linking resin and the photo-acid generator is exposed to the KrF eximer laser of 248 nm or ArF eximer laser of 193 nm, the circuit pattern is inversely tapered by the light absorption of the novolak binder resin and the melamine cross-linking resin (Jour. Vacuum Science Technology., Vol. B7, 1988). Therefore, a research has been performed on a chemically amplified positive photoresist composition tp replace the chemically amplified negative photoresist composition (Pro. Spie., Vol 1262, p32, 1990). The chemically amplified positive photoresist composition contains a photosensitive polymer, a photo-acid generator and a solvent. The photo-acid generated from the photo-acid generator at an exposed part of the photoresist film acts as a catalyst which promotes a depolymerization or deprotection of the main chain or the protection group substituted to the main chain of the photosensitive polymer, so the positive patterns are formed in the succeeding developing process. As the chemically amplified positive photoresist composition, is usually used a resist material containing a polymer such as a polyvinylphenol derivatives which does not absorb the light of 300 nm or less wavelength. Specifically, exemplary chemically amplified positive photoresist composition includes a) the photoresist material comprising a poly(hydroxy styrene) blocked with tertiary-butoxy carbonyl(t-BOC) groups and an onium salt, provided by H. Ito et al. (“Polymers in Electronics”, ACS Symposium Series, No. 242, American Chemical Society, Washington, D.C., 1984, p. 11), b) the photoresist material comprising poly(p-styreneoxytetrahydropyranyl) and the photo-acid generator, provided by Ueno et al. (36th Applied Physics Society Related Joint Meeting, 1989, 1p-k-7) and c) the three-component photoresist material consisting of a novolak resin, t-butoxycarbonyl protected bisphenol-A as dissolution inhibitor and pyrogallol methanesulfonic acid ester, provided by Schlegel et al. (37th Japanese Applied Physics Society announcement, 28p-ZE-4, 1990). The technology related to the preparation of the above-mentioned material is disclosed in Japanese Examined Patent Publication No. 1990-27660, Japanese Unexamined Patent Publication No. 1993-232706, Japanese Unexamined Patent Publication No. 1993-249683, U.S. Pat. Nos. 4,491,628 and 5,310,619 etc. The above-mentioned photoresist composition has a high resolution but a footing phenomenon in the patterns may occur due to the reaction with a substrate when there is a post exposure delay (PED) between exposure process and post-exposure-bake (PEB) process.